1. Field of the Invention
This invention relates to an inspection device using electromagnetic induction for detecting an abnormality in an object to be inspected, and more particularly, to a device for inspecting given objects of all kinds such as foods, medicinal tablets, synthetic resin products and various workpieces, using a change in electromagnetic induction to detect foreign matter or defects in the object.
2. Description of the Prior Art
Magnetic flux changes with the existence of an object in an electromagnetic field, thereby changing the inductance of a coil placed in the electromagnetic field. The inductance of the coil changes proportionally with the dielectric constant, magnetic permeability, size, relative position and other possible inductive factors of the object located in the electromagnetic field. When some of the inductive factors of the object located in the electromagnetic field are known, the other unknown factors can be exactly calculated from the detected change of the inductance. A variety of non-destructive inspection devices using such principles of electromagnetic induction are known for recognition of the existence or identification of given objects.
One of the typically known inspection devices of this type is shown in FIG. 1. This inspection device comprises an electromagnetic coil 1 (self-induction coil) which generates an electromagnetic field with an alternating current and is provided in one arm of a bridge circuit 2. In the bridge circuit, a pair of resistors R1 and R2 are equal in impedance to each other. When the coil 1 is excited in the normal state in which no substance exists in the electromagnetic field induced by applying an alternating current from a power source 3 to the coil, the inductance of coil 1 is in inductance equal in inductance to an adjacent inductor L, and therefore, the bridge circuit 2 on the whole is in equilibrium. In this steadily balanced state, a measuring instrument 4 (usually a galvanometer or sensitive microammeter) connected to diagonal output points P1, P2 of the bridge circuit 2 generates no output (Vout=0).
However, when a object S is located in the electromagnetic field induced by the coil 1, the self-inductance of the coil 1 varies with the coefficient of induction of the object S, thereby breaking the balanced state of the bridge circuit, resulting in an output voltage Vout across the output points P1 and P2.
By analyzing the change in output voltage appearing from the bridge circuit, the physical properties, size, location and other specific features of the given object S to be inspected can be recognized accurately. Furthermore, it is even possible to determine the speed of the object S moving in the electromagnetic field induced by the coil. Also foreign matter or impurities possibly contained in the given object can be detected by comparison with a standard specimen equivalent to the given object. In this case, the standard specimen is arranged in the electromagnetic field induced by the inductor L, so that the bridge circuit 2 assumes its balanced stat when a faultless object is placed in the electromagnetic field induced by the coil.
The aforementioned inspection device is commonly called a "self-induction type" inspection device.
Another type of known inspection device uses mutual induction as shown in FIG. 2. This inspection device comprises an exciting coil 6 (primary coil) excited by a power source 5 to generate an electromagnetic field and a pair of detection coils 7a, 7b (secondary coil) which acquires the electromagnetic field (magnetic flux) generated by the exciting coil 6 and inducing an electromotive current, and a differential amplifier 8. The detection coils 7a, 7b are wound in opposite directions to each other and are differentially connected in series, so that the electromotive currents induced in the respective detection coils 7a, 7b by the electromagnetic field from the exciting coil 6 cancel each other in the normal equilibrium state. That is to say, in the equilibrium state of the detection coils 7a and 7b, the differential voltage across the output points P1 and P2 becomes zero, i.e. Vout=0.
In general, the mutual induction inspection device has an inspection path 9 between the exciting coil 6 and paired detection coils 7a, 7b for allowing a testing object S to pass therethrough across the magnetic flux f brought about by the exciting coil 6. When passing the testing object S through the path 9 across the magnetic flux induced by the exciting coil 6, the magnetic flux which reaches the detection coils 7a, 7b undergoes a change in interlinkage. Namely, the paired detection coils 7a, 7b respectively acquire different interlinkage numbers of the magnetic flux, to thereby break the balanced state of the detection coils 7a, 7b (nonequilibrium state). As a result, a differential voltage Vout is generated from the differential amplifier 8. Thus, it is possible to recognize the quantity and size of the object S or to detect a defect in the object S.
In the prior art inspection devices noted above, since the nonequilibrium state in electromagnetic induction is determined using the differential voltage derived from the bridge circuit 2 or the series connected detection coils 7a, 7b, the change in induced electromotive current brought about by passing the object across the magnetic flux must be detected with a remarkably high accuracy in order to increase the measurement accuracy.
In the self-induction inspection device illustrated in FIG. 1, however, because the rate of change in self-induction (difference between the base inductance and the inductance undergoing a change) is very small, it has been substantially impossible or difficult to accurately detect such a small change in inductance. Thus, the conventional inspection device of the self-induction type has a low sensitivity and can not be applied to inspect measuring objects having a low rate of change in inductance and nonmetallic objects such as of synthetic resin.
On the other hand, the mutual induction inspection device has the inspection path 9 located between the exciting coil 6 (primary coil) and paired detection coils 7a, 7b (secondary coil). The induction efficiency is in inverse proportion to the dimension of the inspection path 9 (distance d from the exciting coil to the paired detection coils). This device is disadvantageous in that the inspection path 9 is limited in dimension from the standpoint of performance and adds to the size in total system size and prevents a large measuring object from passing therethrough.
Even if the inspection path is widened for permitting such a large object to pass therethrough, the inspection accuracy would be decreased proportionally and a slight change in induction could not be detected.
Moreover, the inspection device of the mutual induction type inevitably has a fatal disadvantage in that, when the object S approaches one of the paired detection coils (coil 7a in FIG. 2), not only the coil 7a but also the coil 7b is affected by the object S to cause the coil 7b to vary in inductance. Though either of the detection coils should have, as a reference inductor, a fixed inductance relative to the other coil close to the object S, both the coils vary in inductance even when the object S approaches one of the detection coils. Namely, the induction of the detection coil remote from the object S varies in a complicated manner with the relative position of the object S to the detection coils. Therefore, a change in inductance of one of the detection coils cannot be determined accurately, resulting in a conspicuous decrease in measuring accuracy.
As noted above, the conventional inspection devices using electromagnetic induction are restricted in the size of the object to be inspected and inevitably lead to noticeable measurement errors.